Board computer showing odd numbers of fuel economy

Hey guys, after I bought a used A3 8P 2.0 TFSI Sportback couple of months ago, I had few things repaired. Vacuum pump was leaking oil and needed new gaskets, G12 coolant was changed, oxygen (Lambda) sensor was faulty, and was replaced with used but well preserved one from same A3. Now everything runs smooth except one thing, my board computer is showing a crazy high AVERAGE fuel consumption every time I start a car from cold start. Usually it varies around 11-13 l/100km since I mainly drive in town and 7,8-9 when driving urban. Now when I start a car from cold start it jumps to 37-40 l/100km and it starts decreasing slowly as I drive. It takes about 10-15 mins to chill down to my regular fuel consumption (10-11 l/100km), and it's normal until the next day and next cold start. Fuel consumption is about the same as before, I haven't noticed any difference with my wallet, but why is it's reading messed up like that. When I floor it in red line (S mode) it never goes past 30 l/100km (current), why should average be so high? It doesn't make any sense. And since I usually drive a short routes, till it settles to normal I already arrived at my destination, which means I'm not having a proper readings on my computer at the time and it bothers me. Could it be an oxygen sensor that I replaced that's messing up a computer? Mechanic says when you start from a cold start it's normal to expect richer fuel mixture and above 1000 rpm idle for couple of seconds and I'm aware of that, but it wasn't like that before repairs I mentioned above. Changing Lambda (O2) sensor fixed a strong unburned fuel smell I had in my cabin before and should improve fuel economy, which it probably did, but this problem appeared and now I'm curious if it's related. Maybe a computer needs to recalibrate with WAG-COM or something...don't know really. Let me hear your thoughts, thank in advance!

EDIT: Ok guys it seems like I discovered what's behind this. It was my lack of knowledge after all, because Audi has 2 computer settings, one of which was selected to calculate fuel economy from point of ignition onward, and the other calculates average fuel consumption in general. So that's why it was so high on engine startup (rich fuel mixture due ignition), and it decrease as you drive. And when car is turned off and on it starts from beginning all over again. There is a little indication number in top right corner of the screen showing which settings are currently set (1 or 2).